Rock cutting structures are commercially manufactured using a supporting structure which may be cast, forged and machined. The supporting structure supports rock cutting elements which may be formed either as an integral part of the supporting structure, or as a separate element that is joined to the supporting structure by being forced into an undersized retaining bore formed by, and within, the supporting structure to effect a tight interference fit therewith.
Various sequencing and scheduling strategies are resorted to in manufacturing in order to avoid heat treatments after the cutting elements are pressed into place. This avoids thermal relaxation of the induced stresses provided by the pressing operation. Such a reduction of induced stresses would result in an unacceptable reduction in the retention or holding force provided by the interference fitting operation.
The steels commonly used to produce rock bit cutting structures are graded, which carburize or nitride readily, thus providing a relatively soft core with a hard wear-resistant skin. To form this hard skin on the inner surface of the retention bores would seriously interfere with the installation of the teeth therein, so generally the supporting structure is first carburized, then the carburized surface is machined away in the locations intended for retention bores, then hardened with a heat treatment before the retention bores are machined.
A conventional three-cone rotary rock bit has, typically, from about 100 to about 300 inserted teeth, each of which is carefully fit to provide about 0.004 inch interference fit. Tests indicate that about 0.001 inch interference fit remains as stored stress within the assembly after the pressing, the rest being lost to shearing, galling, and yielding of the steel of the supporting structure. The roller cones and teeth are washed before insertion of the teeth, and no lube is used to install them—they are press fit only. Thus, no fluid is intentionally left behind in the pockets when the teeth are installed.
The irregular heavy impact loads imposed upon the rock bit assembly during drilling tend to cause further yielding in the supporting structure with the subsequent enlargement of retaining bores, and, occasionally, the resultant loss of hard-metal rock cutting teeth within the well bore. Such a lost tooth is no longer operational as a cutting device against rock, but does constitute a source of considerable damage or fracture to the remaining teeth in the rock bit. Serious damage can also occur as a result of a dislodged tooth becoming jammed between cones, or between a cone and the body of the rock bit, thereby interfering with the rotation and cutting action of the cones involved, and of the bit. When cone rotation ceases, a skidding action occurs between the well bore bottom and the cone or cones, and a stopped cone quickly causes self-destruction of the cutting apparatus. Thus, an improved system, a method and apparatus for enhanced cutting element retention and support in a rock bit would be desirable.